Eyes contain a variety of structures with some dimensions near those of the wavelengths of light. Photoreceptor organelles such as rhabdomeres and rod and cone outer segments, myoid inner segments and associated shielding pigment granules are examples. The overall objective and long-term goal of the proposed research is to understand the optical and the physiological functions of such structures. The objective is to explain how the size, shape and refractive index of the photoreceptor organelle and associated structures influence the function of photoreceptor cells. During light and dark adaptation, changes occur in shielding pigment position and in the size, shape and refractive index of both photoreceptor organelles and associated structures such as myoids. The proposed research will focus on possible mechanisms of such photomechanical changes and their optical and physiological consequences.